02 October 2014
the assessments are covered under Chapter XIV of the Income tax Act covering section 139 to 158.
The general assessment is covered under section 143. Where your case is picked for scrutiny, notice under section 143(2) is issued to you for initiating the assessment proceedings. The AO can direct you to furnish documents/information in support of the incomes/expenses reflected in the tax return. At the completion of proceedings AO issues order 143(3).
AO can make additions to your income where deemed fit.
Other forms of assessments like best judgment assessment, reasessment etc are also covered in this chapter.